Abstract: A monitoring system for use as an intruder alarm. The system which is designed to reduce the likelihood of generating false alarms, e.g. alarms generated in the absence of an attempt to break in to premises protected by the system but in the presence of some other occurrence, includes a sensor circuit having at least one set of normally-closed contacts coupled in series between terminal means for coupling the circuit to a source of current and output means coupled to the sensor circuit for providing, in use, a first output signal upon actuation of a set of contacts for a predetermined time interval the length of which may be varied as desired. The system includes pulse generating means for providing the first output signal in the form of pulses, timing means for defining said interval and counting means for counting the output pulses during the said interval and for providing a further output signal when the number of pulses counted in the said interval equals or exceeds a predetermined value.